The present invention relates generally to attachment of airfoil blades in a gas turbine engine and more particularly to apparatus for radially preloading fan blades in a turbofan engine.
Existing turbofan engines include those having fan blades with dovetail roots which are inserted into corresponding dovetail slots in the fan disk of the engine. To allow for easy insertion and removal of the fan blades, the attachment arrangement necessarily requires that there be a radial space between the bottom of the engaged root of each fan blade and the bottom surface the corresponding slot in the fan disk. This results in there being some play between an attached fan blade and the fan disk. When the engine is operating, the fan blades are rotated at high speeds such that centrifugal force pushes the fan blades radially outward whereby the blade root is tightly engaged in the dovetail slot of the fan disk. However, when the engine is not operating, the fan blades are free to slowly rotate or windmill due to wind or breezes on the ground at the airport. Under such slow and varying rotation, the root of the attached fan blade rubs against and wears on the corresponding dovetail slot of the fan disk.
U.S. Pat. No. 3,936,234 preloads the fan blade with a radially outward force to eliminate wear between the blade root and the dovetail slot during windmill conditions. In that patent, the fan-blade-to-rotor attachment/locking device includes a spacer member 32 disposed between the blade tang (root) and the bottom of the rotor (disk) slot, the spacer member 32 having a bottom recess 70 containing a biasing wedge 80 comprised of resilient material capable of elastic deformation. The biasing wedge 80 has an axial length which is too long for bottom recess 70 so that its middle bulges out of the recess (see FIG. 9). However, when the biasing wedge 80 is radially compressed between the bottom of the rotor slot and the spacer member, in the blade-locked position, it will bias the blade radially outward (see FIG. 10).
U.S. Pat. No. 4,208,170 also preloads the fan blade with a radially outward force to eliminate wear between the blade root and the dovetail slot during windmill conditions. In that patent, the fan-blade-to-rotor attachment device includes a spacer 52 made of a high-strength metallic alloy which acts as a spring. The spacer 52 is disposed between the blade tang and the bottom of the rotor slot to bias the blade radially outward.
The blade preloading apparatus disclosed in the above-described patents are not adjustable, meaning they cannot be adjusted during routine inspection to maintain a constant preload force against the root of the fan blade as the spring spacer of U.S. Pat. No. 4,208,170 and the resilient biasing wedge of U.S. Pat. No. 3,936,234 lose their resiliency over the operating lifetime of the fan blades in the engine.